1. Field of the Invention
The present invention relates to an image processing method of a hold type display device and a driving method therefor and a display device using the driving method, and more particularly to an image processing method and a driving method of a display device and a display device using the driving method, for improving the image quality of a moving picture (moving picture quality).
2. Description of the Related Art
In recent years, the size of a display screen, the display precision and the purity of unmixed color have been increased in an active matrix type liquid crystal display device, so that a still image with sufficiently high image quality can be displayed. On the other hand, although in displaying a moving picture, the image quality has been improved by increasing the response speed of liquid crystals, an image quality equivalent to CRT (Cathode Ray Tube) has not yet been obtained.
When a moving picture display is performed by a hold type display device including a liquid crystal display device, the contour of a moving object is visually recognized as blurred for the observer who watches the display object moving on the screen, so that the moving picture quality is recognized to be lowered (hereafter, a phenomenon (blurring of moving picture) in which the contour of a display object is visually recognized as blurred due to the movement of the display object on the screen is noted as “edge blurring”).
Causes of such deterioration of the moving picture quality in the hold type display device are explained in detail in Ishiguro, Kurita, The Institute of Electronics, Information and Communication Engineers technical research report, EID 96-4 (1996) (hereinafter referred to as Non Patent Document 1). It is described in Non Patent Document 1 that the deterioration of the moving picture quality in a liquid crystal display device is, in principle, caused by the 0 order holding (continuously displaying the same gradation within one frame period) in an active element, such as TFT (Thin Film Transistor).
This indicates that the deterioration of the moving picture quality cannot be prevented only by increasing the response speed of liquid crystal in the liquid crystal display device. That is, the deterioration of the moving picture quality is caused by the 0 order holding of a display element, and cannot be avoided by the conventional driving method.
The deterioration of the moving picture quality can be improved by increasing the rewriting speed (frame frequency) of a picture, but in this method, originally non-existent frame pictures (pictures displayed between the original frame pictures) need to be interpolated by image processing, as a result of which it becomes difficult to improve the deterioration of the moving picture quality by this method. When the frame frequency is set high, the amount of data at the time of transmitting a video signal is increased, which makes it impossible to apply the method to existing broadcast facilities in which the capacity of transmission lines for video signals is not ensured sufficiently.
In order to solve the above problems, several methods have been proposed, in which a liquid crystal having a high speed response characteristic is used to perform black resetting within a frame (displaying black in the pixel without regard to its original gradation value during a predetermined period within one frame), thereby realizing a pseudo impulse type display for improving the moving picture quality.
Methods for performing the black resetting include a (black reset driving) method of writing in a liquid crystal a reset voltage corresponding to the black output (the first black resetting method), a method of flashing the backlight synchronously with the frame period (the second black resetting method) and a method of using an optical shutter moving in the same direction as the scanning direction of driving (the third black resetting method). Conventional techniques relating to the first black resetting include a “display device” disclosed in Japanese Patent Application Publication No. 2000-122596 (page 6 to 7, FIG. 7) (hereinafter referred to as Patent Document 1) and a “display device” disclosed in Japanese Patent Application Publication No. 2002-23707 (page 4 to 5, FIG. 6) (hereinafter referred to as Patent Document 2). Conventional techniques relating to the second black resetting method include a “liquid crystal display device” disclosed in Japanese Patent Application Publication No. 2000-275604 (hereinafter referred to as Patent Document 3). Further, conventional techniques relating to the third black resetting method include a “projection type liquid crystal display device” disclosed in Japanese Patent Application Publication No. 2002-148712 (hereinafter referred to as Patent Document 4).
The invention disclosed in Patent Document 1 provides a display surface having a plurality of pixel lines, where the display surface is configured such that during a period of writing an image into at least one of the plurality of pixel lines, black color is written in other pixel lines to enable the black resetting to be performed, thereby improving the moving picture quality.
The invention disclosed in Patent Document 2 provides a hold type display device, in which a frame, serving as a unit time for displaying a picture, is time-divided into a plurality of sub frames, and in which the luminance of a picture inputted to the device itself is decreased at a predetermined rate in accordance with the luminance of a previously inputted picture. The employment of such configuration of the invention disclosed in Patent Document 2 prevents a picture from becoming blurred or obscure in displaying a moving picture, while suppressing the lowering of the luminance of a picture.
The invention disclosed in Patent Document 3 provides a liquid crystal display device in which an illuminator having a plurality of lamps is divided, and after a fixed time period from the time when a response is made by a liquid crystal display section, each of which corresponds to each divided area of the illuminator, lamps of the illuminator in the area corresponding to the responded area are controlled to be turned on by an illumination driver and then after a fixed time period to be turned off. Such configuration decreases the edge blurring due to the 0 order holding, thereby enabling the moving picture quality to be improved.
The invention disclosed in Patent Document 4 provides a configuration in which a mechanical or electric shutter is arranged in the optical path, and opened and closed in sync with one field of the display picture so as to cut off non-stationary parts of the display light. Such configuration decreases the edge blurring due to the 0 order holding, thereby enabling the moving picture quality to be improved.
However, each method for preventing the edge blurring by inserting the above described black resetting, which is capable of suppressing deterioration of the moving picture quality resulting from the 0 order holding, causes another problem in which the displaying luminance and the contrast are lowered by inserting the black resetting.
In particular, the application of the techniques according to the inventions disclosed in the above described Patent Documents 1 and 2 lowers the luminance at the time of displaying white color, which has the maximum luminance.
In the invention disclosed in the above described Patent Document 3, the reduction in the display luminance at the time of displaying a still image is suppressed by making all the light sources of the illuminator into a lighted state, but at the time of displaying a moving picture, the luminance level is lowered compared to the case where the black resetting is not performed, as in the case of the inventions described in the Patent Documents 1 and 2.
The invention disclosed in the above described Patent Document 4 allows the black resetting to be performed only with the entire screen of the display device or with one line as a unit. As a result, at the time of displaying a moving picture, pixels without the need of being black reset are made to be black reset, so that the display luminance is lowered.
In this way, hitherto, it has been impossible to improve the moving picture quality without decreasing the maximum luminance and the contrast.